The present invention relates to fabrication of gas turbine engine components and the like, and more particularly, to a novel method for preparing the surface of a gas turbine engine component or the like, for finishing, coating or bonding using an ultraviolet laser.
It is known that the roughness of a surface may be an important constituent in a bonding or coating operation. The surface texture or roughness provides a "mechanical key" for the coating material or adhesive used in the bonding operation, to facilitate the attachment of the coating material or bonding material layer to the parent material and to insure the structural integrity of the finished component.
Conventional methods of surface preparation, used in heavy industry, such as gas turbine engine manufacturing, basically include mechanical processes, such as grit blasting, honing, grinding and the like. Each of these methods involves contacting a surface with an abrasive medium; these methods typically are not easily controlled for very precise surface preparation, and would not generally be employed where a particular pattern is desired for forming a mechanical key. Conventional surface preparation methods can also introduce undesirable deformation or other damage into the surface or substrate; the abrasive material may become embedded in some parent materials or otherwise leave behind particulate contaminants or residue which will require an additional process step to clean the surface and remove any contaminants or residue.
The preparation and texturing of gas turbine engine components for subsequent bonding or coating operations is typically performed by grit blasting with aluminum oxide, followed by washing in acetone. This procedure frequently leaves small aluminum oxide particles embedded in the roughened surfaces. These embedded particles can have a detrimental effect on the integrity of the subsequently established bond between the bonded layer, or coating, and the component and can cause premature fatigue or failure of the component.
Preparation of surfaces for subsequent bonding or coating operations is becoming even more critical as nonmettallic composite materials, such as PMR-15 (a carbon or glass fiber composite with an organic amide resin), find greater application in devices subjected to high stress and heat, such as the components of a gas turbine engine or the like. Conventional surface preparation methods are generally inappropriate for composite-type materials for the same reasons previously discussed, and variations in the condition of the surface of these materials from one batch to another and between different suppliers may require special surface preparation before subsequent bonding or coating process steps to ensure uniformity and reliability of the finished component.
Components made of a composite material are typically molded. The unformed composite material is coated with a mold release agent so that it can be easily extracted from the mold after formation into the desired component. The mold release agent must be completely removed from the surface of the component to insure proper adhesion in a subsequent coating or bonding operation. Surface preparation is therefore particularly important in the fabrication of composite gas turbine engine components to insure reliability and a long service life.
The use of excimer lasers (ultra-violet lasers) for material processing, such as micro-machining and deposition of metallic and insulator films is described in marketing and technical literature published by LAMBDA PHYSIK GmbH, a leading manufacturer of excimer lasers, whose address is Hans-Bockler-Str. 12, D-3400, Gottingen, Federal Republic of Germany, and in U.S. Pat. Nos. 4,617,085; 4,756,785; 4,720,621; 4,368,080; 4,328,410; 4,219,721; and 4,128,752. None of these documents recognize the specific problems, as mentioned hereinabove, associated with preparing the surface of a gas turbine engine component, and in particular a component made of a composite material such as PMR-15, for a subsequent bonding or coating operation during the fabrication of a gas turbine engine component.
A related invention, disclosed in U.S. Pat. No. 5,120,395, entitled "A METHOD FOR MAKING A GAS TURBINE ENGINE COMPONENT WITH A TEXTURED SURFACE", and assigned to the same assignee as the present invention, discloses a method for selectively patterning a surface of an unfinished engine component by focusing a pulsed UV laser on selected locations of the component surface to form micro-pores therein. The micro-pores act as tiny lubricant reservoirs during engine operation to improve the distribution of lubricant between the patterned engine component surface and the surface of another engine component in contact with the patterned surface during engine operation.